I had a Dahon Mu that uses a Neos rear derailleur. The upper/guide pulley is a 9-tooth pulley and one of the teeth was broken. I thought I could go out and nab one quickly but didn't find them anywhere. I was left with the option of buying a new derailleur or trying to use another make/model derailleur - so I 3D printed some. I was experimenting a little, so I made a bunch of them. They have worked great on my Dahon. I have a few options if you are in the same situation. From hardest to softest they are:

1. PA12CF
2. PA12CF, all walls
3. PETG-CF (all walls)
4. PET-G
5. PET-G (all walls)

I printed in different orientations and haven't had a problem with them. If you try it on your bike and it doesn't work, you can ship back to me for a refund, but the testing I have done suggests they work just fine. Hope this can solve some problems for others. Thanks!